Accelerated synthesis of MnO₂ nanocomposites by acid-free hydrothermal route for catalytic soot combustion

Abstract

MnO₂ nanocomposites with porous structure were successfully synthesized by a facile hydrothermal route from KMnO₄ without the addition of any acid. Among CeO₂, CuO and Co₃O₄, an accelerated synthesis of MnO₂ assisted by CeO₂ was observed, which contributed to the redox transformation between Ce³⁺ and Mn⁷⁺. The as-prepared catalysts were characterized by XRD, N₂ adsorption–desorption, ICP, SEM, TEM, H₂-TPR, and XPS. Lattice shrink occurred after the hydrothermal reaction of CeO₂ with KMnO₄, while lattice expansion in CeO₂ was observed after calcination at high temperature. The above phenomena are ascribed to the conversion between small sized Ce⁴⁺ and large sized Ce³⁺ in CeO₂. The concentration of surface Ce³⁺ decreases after the hydrothermal reaction with KMnO₄. The greater proportion of Ce³⁺ after calcination results in more oxygen vacancies that are beneficial for the catalytic oxidation of soot. The best catalytic performance is acquired over the CeMn-600 catalyst and the corresponding T₁₀, T₅₀, and T_m,l are 320 °C, 418 °C and 420 °C, respectively. The strong interaction between the CeO₂ and MnO₂ is also a key factor for outstanding catalytic activity for soot combustion.

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1. Introduction

Nanocomposite materials are good candidates for catalysts with high activity and selectivity owing to their multicomponent composite and special morphology. Among them, MnO₂ nanocomposites have attracted significant interest not only for their unique catalytic, electrochemical, magnetic, and adsorptive properties, but also their low toxicity and cost.^1,2 In nature, MnO₂ exhibits different crystallographic forms, such as the α, β, γ, and δ types, depending on how the MnO₆ octahedra units are interlinked with each other.² Among them, α- and δ-MnO₂ have been frequently applied as electrode material and catalyst.^3–7 Nevertheless, the crystalline phase and morphology of MnO₂ are also highly dependent on the precursors and preparation methods.⁴ Different synthetic strategies have been developed for the preparation of MnO₂, such as co-precipitation,⁸ sol–gel,⁹ microemulsion,¹⁰ hydrothermal,^7,11–14 and electrochemical methods.¹⁵ It is significant to develop a facile way to synthesize MnO₂ with nanostructure.

Hydrothermal routes are considered as convenient and effective synthetic techniques for the preparation of inorganic compounds.¹⁶ The main advantage of hydrothermal techniques over other routes is the ability to control the nanostructures by proper choice of the reaction temperature or time as well as the solvent used for the reaction. The factors mentioned above can be judicially chosen to obtain the desired architecture. Crystalline MnO₂ was prepared by a hydrothermal route at 170 °C for 4 days, using KMnO₄ solution as starting material under acidic conditions.¹⁶ The reaction mechanism can be described as follows: 4KMnO₄ + 2H₂O → 4MnO₂ + 4KOH + 3O₂. But it's a pity that this reaction takes too much time and the yield is very low.¹⁷ Plate- and rod-like MnO₂ were synthesized by the hydrothermal reaction of KMnO₄ and MnSO₄.¹⁸ Recently, MnO₂ with flower-like morphology was synthesized by a microwave-assisted hydrothermal method from an acidic solution consisting of KMnO₄.¹² Composite nanomaterials often exhibit superior performances over those of individual nanomaterials in various fields, such as catalysis and so forth.^19–24 For example, size- and shape-controlled synthesis of porous CuO–MnO₂ nanocomposites toward efficient nitroarene reduction were fabricated via a facile and surfactant free redox transformation approach.¹⁹ Generally, acid medium has been used to accelerate the hydrothermal process of KMnO₄, which inevitably brings serious environmental problems. Thus, it's urgent to find a way that both can accelerate the reaction of KMnO₄ and also improve catalytic properties in the meantime.

In this work, three types of MnO₂ nanocomposites with controlled morphology were synthesized hydrothermally from KMnO₄ without the using of any acid or template agent. The effects of CeO₂, CuO and Co₃O₄ addition on the formation of MnO₂ nanocomposites were investigated in detail. The XRD, N₂ adsorption–desorption, SEM, TEM, H₂-TPR and XPS were performed to characterize the structure and redox ability of the MnO₂ nanocomposites. Considering the multiple oxidation states and outstanding redox properties of MnO₂, diesel soot combustion was used to evaluate the catalytic efficiency of the MnO₂ nanocomposites.

2. Experimental details

2.1. Catalyst preparation

The synthesis of pure MnO₂ was carried out by a facile hydrothermal method using aqueous solutions of KMnO₄ as starting material without the addition of any acid or surfactant, and the procedure was briefly described below. 60 mL KMnO₄ (0.05 M) aqueous solution was transferred to a Teflon-lined stainless steel autoclave and heated under 160 °C for 24 h. The obtained product was filtered and washed with distilled water and ethanol for several times. After dried at 60 °C overnight, the above product was calcined at 600 °C for 5 h. The MnO₂ obtained before and after calcinations were denoted as Mn-0 and Mn-1, respectively.

Three metal oxide nanocomposites, CeO₂–MnO₂, CuO–MnO₂ and Co₃O₄–MnO₂, were prepared by a one-step hydrothermal route. In a typical synthesis, 0.1 g of CeO₂ was dispersed into the 60 mL of 0.05 M KMnO₄ solution with vigorous stirring. Then, the above mixture was transferred to a Teflon-lined stainless steel autoclave, and heated at 160 °C for 24 h. The resulting product was washed with distilled water and ethanol, and dried at 60 °C overnight. The as-prepared samples were donated as CeMn, CuMn and CoMn, respectively. Finally, the above samples calcined at 600 °C for 5 h were marked as CuMn-600, CoMn-600 and CeMn-600, respectively.

2.2. Catalyst characterization

X-ray diffraction (XRD) patterns were recorded on a SmartLab-9 Japan automated power X-ray diffraction meter operating at 100 mA and 40 kV using Cu Kα (λ = 0.1541 nm) radiation. The data of 2θ ranged from 5° to 80° were collected with a step scan of 0.02°. The surface area and pore size distribution of the as-prepared samples were determined by N₂ adsorption–desorption isotherms at liquid N₂ temperature (77 K) on a Micromeritics (ASAP 2000) analyzer. Specific surface area and pore size distribution were calculated by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Prior to the measurements, the samples were pretreated in a vacuum oven at 473 K for 3 h to remove any residual moisture. Scanning Electron Microscopy (SEM) was taken on a Hitachi S4800 field-emission SEM instrument operated at 5 kV. Transmission electron microscopy (TEM) images were obtained using a JEM-100CX electron microscope. The catalyst was ultrasonically dispersed in ethanol and the suspension was dropped onto holey carbon coated copper TEM grid. X-ray photoelectron spectroscopy (XPS) is an excellent technique to understand the oxidation state of the transition metal ion and the relative composition of the synthesized material. XPS spectra were recorded on a PHI-5000 spectrometer using Al Kα (1486.6 eV) radiation as the excitation source. All binding energies were referenced to the C 1s peak at 284.5 eV, Gaussian–Lorentzian and Shirley background was applied for peak analysis. H₂ temperature-programmed reduction (TPR) was conducted on a Micromeritics AutoChem II 2920 instrument equipped with a TCD detector. Each time, 50 mg sample was pretreated under He stream at 200 °C for 30 min. After cooling down to room temperature, 5 vol% of H₂/Ar with a flow rate of 30 mL min⁻¹ was passed over the catalyst, the temperature was raised to 800 °C at a ramp of 5 °C min⁻¹. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was performed on the Perkin Elmer instrument to determine the content of metals in the catalysts.

2.3. Activity test

Temperature programmed oxidation (TPO) test under the loose contact (LC) mode was used to evaluate the activity of different catalysts on the continuous fixed-bed reactor. Printex-U from Degussa was used as model soot and mixed with the catalyst in a weight ratio of 1 : 9 in an agate mortar for 30 seconds. The above mixtures were placed into the quartz tube (i.d. = 14 mm) and heated from 200 to 700 °C at 2 °C min⁻¹ heating rate in a flow of feed gas (21% O₂ and 79% N₂) with a flow rate of 100 mL min⁻¹. The outlet gas compositions were analyzed by the infrared gas analyzer (Infralyt 50). In this work, the temperatures at 10%, 50% and 90% conversion of soot were denoted as T₁₀, T₅₀ and T₉₀, respectively. The temperature for maximum CO₂ concentration (T_m,l) under the light contact mode was also used to evaluate the catalytic performance of the catalysts.

3. Results and discussion

The weight of the prepared metal oxides under different times is shown in Table 1. When no metal oxide is added to the KMnO₄ solution, only 0.0233 g MnO₂ is formed in 24 h, and the amount of the obtained MnO₂ only increases slightly after prolonging the hydrothermal synthesis time to 48 h. The same phenomenon is observed when 0.1 g CuO or Co₃O₄ is added for 24 h of hydrothermal reaction. However, the formation of MnO₂ is greatly accelerated when CeO₂ is added, as 0.3676 g CeO₂–MnO₂ mixed metal oxides is obtained in 24 h, the weight of MnO₂ is calculated to be 0.2676 g as determined by ICP-AES studies. It should be noticed that the promoted formation of mixed metal oxides is also perceived for Co₃O₄ by extending the hydrothermal synthesis time to 48 h. The above phenomena are probably due to the oxidation of low valent metal ion by Mn⁷⁺ with strong oxidizing property. Both CeO₂ and Co₃O₄ could significantly promote the formation of mixed metal oxides due to the oxidization of low valent Ce³⁺ and Co²⁺ by Mn⁷⁺. By comparison, Cu²⁺ with high valence can't be oxidized by Mn⁷⁺, thus the addition of CuO couldn't enhance the formation of MnO₂ even in 48 h.

Table 1 Weight and composition of the as-prepared MnO₂ nanocomposites before calcination

Sample	W^a (g)	W^b (g)	Mn content^c (wt%)	W^d (g)
a 160 °C for 24 h.b 160 °C for 48 h.c Determined by ICP-AES (samples for 24 h).d Weight of raw CuO, Co3O4 or CeO contained in mixed oxides calculated by ICP-AES.
Mn-0	0.0233	0.0283	—	—
CuMn	0.1305	0.1354	19.1	0.0999
CoMn	0.1865	0.3825	42.6	0.0998
CeMn	0.3676	0.3702	67.4	0.0999

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XRD is used to characterize the structure of the as-synthesized samples and metal oxide nanocomposites. The XRD patterns of the samples obtained in 24 h before and after calcination are displayed in Fig. 1(A) and (B), respectively. As shown in Fig. 1(A), the main diffraction peaks of Mn-0 at 2θ values of 12.4°, 24.7°, 36.2° correspond to the (001), (002) and (111) of δ-MnO₂ (JCPDS 80-1098) with layered structure.²⁵ The feature of broad peak illustrates the obtained δ-MnO₂ are composed both of amorphous and nanocrystalline sample. After the addition of CeO₂, the characteristic diffraction of CeO₂ also appears apart from δ-MnO₂, which illustrates the formation of mixed CeO₂–MnO₂ materials. For CoMn and CuMn mixed oxides, the characteristic peaks of δ-MnO₂ are only detectable as illustrated in Fig. 1(A). As demonstrated in Table 1, the as-prepared mixed metal oxides are mainly composed of the added CuO or Co₃O₄, and the formed δ-MnO₂ phase may be more prone to be amorphous. The character peaks of δ-MnO₂ are stronger for CoMn sample, which is verified by the more formation of CoMn mixed oxides. After calcination at 600 °C for 5 h, the pure δ-MnO₂ phase turn into α-MnO₂ as illustrated by Fig. 1(B), in which the main diffraction peaks agree well with the crystalline structure of α-MnO₂ (JCPDS 44-0141). The transformation of δ-MnO₂ to α-MnO₂ also witness the increase in crystallinity. The XRD patterns of CeMn-600 sample are shown Fig. 1(B), δ-MnO₂ also transform to α-MnO₂ with good crystallinity, while the six peaks indexed to the CeO₂ phase at 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, and 69.4° keep intact, which verifies that the CeMn-600 sample exists in separate phase rather than in the form of Ce–Mn solid solution.^22,26–28 The δ-MnO₂ existed in CuMn and CoMn are also converted to α-MnO₂ with more crystallinity.

Fig. 1 XRD patterns of different metal oxides and composites: (A) before calcinations; (B) after calcinations; (C) Ce-type metal oxides.

As shown in Fig. 1(C), the (111) peak of the CeMn shifts to higher angles compared with that of pure CeO₂, even more special, the diffraction peaks of CeMn-600 interestingly shift back to lower angles compared with pure CeO₂ and CeMn, which meant that the CeMn-600 sample shows a higher lattice parameter compared with that of pure CeO₂ and CeMn. The presence of Ce³⁺ ions could lead to charge imbalance,²⁹ which may create oxygen vacancies and unsaturated chemical bonds in the CeO₂ lattice.^30–32 It has been reported that the conversion of Ce⁴⁺ to Ce³⁺ is a key factor for the expansion of the ceria lattice.^32–35 On the contrary, the conversion of Ce³⁺ to Ce⁴⁺ also led the shrink of the ceria lattice. After calcination at 600 °C, there is an expansion in the ceria lattice during the reduction of Ce⁴⁺ to Ce³⁺ in CeMn-600.

Fig. 2 shows the N₂ adsorption/desorption isotherms and the associated pore size distribution of the MnO₂ nanocomposite catalysts. It's obvious that all the four samples possess the type IV isotherm with a type H₃ hysteresis loop indicating a mesoporous structure. The specific surface areas and average pore size of various samples are presented in Table 2. The specific surface areas and the average pore sizes of the four catalysts are similar, which are all within the range of 12–19 m² g⁻¹ and 22.09–31.79 nm. It reveales that the addition of CeO₂, CuO and Co₃O₄ metal oxides in the synthesis procedure has not resulted in significant changes of the physical properties.

Fig. 2 N₂ adsorption/desorption isotherms and pore size distribution of the MnO₂ nanocomposite catalysts: (A) Mn-1; (B) CeMn-600; (C) CuMn-600; (D) CoMn-600.

Table 2 BET surface area, average pore size, and O_ads/O_lat ratio of the Mn nanocomposite catalysts

Catalyst	Surface area (m^2 g^−1)	Average pore size (nm)	Oads/Olat
Mn-1	17	22.09	0.223
CeMn-600	19	31.79	0.385
CuMn-600	12	27.46	0.172
CoMn-600	16	28.05	0.199

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The SEM images of the as-synthesized samples are demonstrated in Fig. 3. Using only KMnO₄ as starting material, Fig. 3(a) illustrates that δ-MnO₂ nanoflowers consisted of nanoflakes are formed by the facile hydrothermal synthesis method without the assistance of any acid or surfactant, which corresponds to the layered structure of the synthesized δ-MnO₂ as demonstrated by XRD measurements. Fig. 3(c) shows that the morphology of CeMn is similar to that of δ-MnO₂. The addition of CuO and Co₃O₄ also has no obvious effect on the morphology of δ-MnO₂ as illustrated by Fig. 3(e) and (g). However, the particle size become larger compared with pure Mn-0 or CeMn. After calcination at 600 °C for 5 h, the architecture of δ-MnO₂ transforms from nanoflowers to individual α-MnO₂ nanorods as observed in Fig. 3(b). Thus, the morphology of MnO₂ can be manipulated by calcinations. The similar transformation also takes place for the conversion of CeMn to CeMn-600. As illustrated in Fig. 3(f) and (h), larger amounts of particles also appear, which can be ascribed to the unoxidized CuO or Co₃O₄. Because Co₃O₄ can be partially oxidized by KMnO₄, the amount of α-MnO₂ nanorods in CeMn-600 is much larger than that of CoMn-600.

Fig. 3 SEM and TEM images of the MnO₂ nanocomposites: (a) Mn-0, (b) Mn-1, (c) CeMn, (d) CeMn-600, (e) CuMn, (f) CuMn-600, (g) CoMn, (h) CoMn-600.

The morphology of CeMn are further demonstrated in Fig. S1(a)–(d),† which clearly show that the flower-like CeMn samples are composed of nanoflakes. After calcination at 600 °C for 5 h. The nanoflakes of CeMn are converted to nanorods.

With the help of XPS analysis, the chemical state and surface atomic composition of samples are authenticated. To determine the oxidation states of the elements presented in mixed metal oxides, the main core level peaks for Ce 3d, Cu 2p and Co 2p of CeMn, CuMn and CoMn are shown in Fig. 4(B), (D) and (F), respectively. For comparison, the counterpart peaks of CeO₂, CuO and Co₃O₄ are shown in Fig. 4(A), (C) and (E), respectively. The relative concentration of Ce³⁺ and Co²⁺ in the MnO₂ nanocomposites are estimated from the ratio of integrated peaks in Fig. 4, and the results are listed in Table S1.† As noted from the Fig. 4(A), the six binding energy peaks labeled as u, u′, u′′, u′′′, v′, v′′′ are featured to Ce⁴⁺, while the other two peaks labeled as v, v′′ are related to low valent Ce³⁺, which can be oxidized to Ce⁴⁺ by strong oxidants such as KMnO₄.^31,36,37 Thus, the CeO₂ can be used to promote the hydrothermal synthesis of MnO₂ from KMnO₄ by the redox transformation reaction between Ce³⁺ and Mn⁷⁺. After this redox transformation, the binding energy peaks ascribed to Ce³⁺ decreases. Table S1† also demonstrates that the percentage of Ce³⁺ decreases from 19.6% to 12.3% after the redox transformation.

Fig. 4 XPS spectra of Ce 3d, Cu 2p, Co 2p for pure CeO₂ (A) and CeMn (B), pure CuO (C) and CuMn (D), pure Co₃O₄ (E) and CoMn (F), Co 2p of CoMn-600 (S1), Ce 3d of CeMn-600 (S2).

The ratio of Co²⁺ species in CoMn only decreases slightly after 24 h of hydrothermal synthesis, which means that the Co²⁺ species in Co₃O₄ are hard to be oxidized. This result is in accordance with that of Table 1 and XRD, in which the promotion effect of Co²⁺ species is unsatisfactory, thus the amount of the formed δ-MnO₂ is limited. After prolonging the hydrothermal reaction time to 48 h, the ratio of Co²⁺ decreases significantly, and the amount of CoMn increases substantially as shown in Table 1. Thus, extending of time is helpful for the redox transformation reaction between Co²⁺ and Mn⁷⁺, as shown in Fig. 4(S1), the concentration of Co²⁺ ions in Co₃O₄ further decreases in 48 h.

It's noteworthy that the concentration of Ce³⁺ ions in CeMn-600 increases to a high value as demonstrated in Table S1† and Fig. 4(S2). Particularly, the presence of higher numbers of Ce³⁺ could bring about a charge imbalance and facilitate the formation of more oxygen vacancy defects, which are expected to be helpful for their catalytic oxidation performance.

Fig. 5 shows the O 1s spectra of α-MnO₂ and MnO₂ nanocomposite catalysts. The O 1s spectra can be divided into two types of oxygen species using the curve-fitting approach to get a relative content of different oxygen species and the detailed results are illustrated in Table 2. The binding energy peak at 528.5–530.2 eV is assigned to the lattice oxygen species (denoted as O_lat). While the other binding energy peak at 532.4–533.5 eV can be attributed to the surface adsorbed oxygen species (O_ads).³⁸ Generally, the surface adsorbed oxygen species (O_ads) play an important role in oxidation reactions, thus the area ratio of O_ads/O_lat is used to evacuate the activity of catalyst for the oxidation. As is shown in Table 2, the O_ads/O_lat proportion of CeMn-600 (0.38) is much higher than that of α-MnO₂ (0.23), while the O_ads/O_lat proportion of CoMn-600 and CuMn-600 are much lower.

Fig. 5 XPS spectra of O 1s for the MnO₂ nanocomposite catalysts.

Fig. 6 illustrates the H₂-TPR profiles of the catalysts. The reduction profile of α-MnO₂ exhibits two well-resolved reduction peaks centered at 243 and 295 °C, in good agreement with the TPR curves of the bulk MnO₂ samples.^39,40 The former reduction peak is ascribed to the reduction of MnO₂ → Mn₃O₄ and the latter one is due to the further reduction of Mn₃O₄ → MnO.

Fig. 6 H₂-TPR profiles of the MnO₂ nanocomposite catalysts.

According to literature,^31,41 the reduction profile for Co₃O₄ exhibits a broad peak around 317 °C, referring to the reduction processes of Co³⁺ → Co²⁺ → Co, where two reduction steps take place simultaneously. After the formation of CoMn-600 nanocomposites, the main reduction peak of Co₃O₄ shifts to 320 °C with a small shoulder at 300 °C, suggesting the lower reducibility of CoMn-600 after the hydrothermal reaction with KMnO₄, which is ascribed to the simultaneous reduction of Co₂O₃ to Co and MnO₂ to MnO. The H₂-TPR profile CuMn-600 also depicts a main peak at 329 °C, which is attributed to the reduction of CuO.^42,43

The H₂-TPR profile of CeMn-600 shows two reduction peaks at 180 °C and 264 °C, which are corresponded to the typical two reduction steps of MnO₂ → Mn₃O₄ → MnO processes and surface Ce⁴⁺ → Ce³⁺. The first stage is ascribed to the reduction of MnO₂ to Mn₃O₄, and the second stage is ascribed to the combined reduction of Mn₃O₄ to MnO and surface Ce⁴⁺ to Ce³⁺ species. The addition of CeO₂ to manganese oxide facilitates the mobility of the oxygen on the catalyst surface and the reduction of manganese oxide from MnO₂ to MnO.

Therefore, CeMn-600 possesses the best low-temperature reducibility, which may induce excellent catalytic activity for soot combustion. High numbers of Ce³⁺ ions, greater proportion of surface adsorbed oxygen species and superior reducible nature of MnO₂ are discovered for the CeMn-600 nanostructures compared with those of α-MnO₂ and the other two MnO₂ nanocomposites. The above characters are probably due to the strong interaction between the MnO₂ and CeO₂ phases at their interface, where these properties are expected to be existed.³⁷

The prepared MnO₂ nanocomposite metal oxides catalysts are applied to soot combustion due to their special composition and morphology, which might make it advantageous over single metal oxides. The activities of catalysts are analyzed by temperature programmed oxidation. The activity of as-prepared catalysts for soot combustion under the loose contact mode is displayed in Fig. 7. The calculated T₁₀, T₅₀ and T₉₀ values of all samples are summarized in Table 3. For soot combustion without catalyst, the T₁₀, T₅₀ and T₉₀ are 482, 565 and 609 °C, respectively. α-MnO₂ demonstrates good activity with relatively low combustion temperature of soot, which can be ascribed to the porous structure and good contact between α-MnO₂ with nanorod structure and the soot particles. CeMn-600 possess more percentage of surface adsorbed oxygen species (O_ads), thus CeMn-600 shows higher active than α-MnO₂. The activity of CuMn-600 and CoMn-600 are lower than α-MnO₂. As shown in Table 1 and Fig. 3, the promotion effect of CuO and Co₃O₄ on the hydrothermal process of KMnO₄ is not significant, because the active MnO₂ phase in CuMn-600 and CoMn-600 based on weight is low.

Fig. 7 The activity of the MnO₂ nanocomposite catalysts for soot combustion.

Table 3 The temperature for combustion soot over MnO₂ nanocomposite catalysts

Catalyst	T10 (°C)	T50 (°C)	T90 (°C)	Tm,l (°C)
Soot (without catalyst)	482	565	609	—
Mn-1	345	459	502	447
CeMn-600	320	418	493	420
CuMn-600	382	494	536	483
CoMn-600	368	469	513	461

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In fact, the CeMn-600 sample exhibits superior catalytic activity toward soot combustion than bare MnO₂ sample. CeMn-600 demonstrates stronger synergistic interaction between Ce- and Mn-oxides,^44,45 which which could promote the redox properties of the surface oxygen-containing intermediate as supported by XPS results. Further, the surface oxygen vacancies also play a vital role for the formation of “active oxygen” via activating the chemisorbed oxygen or the readily releasable lattice oxygen. Therefore, the catalytic performance is closely related to the oxygen vacancy in the sample. All the above superiorities in physicochemical features of CeMn-600 sample could explain its better performance in soot oxidation activity, which show the T₅₀ and T₉₀ at ∼418 and 493 °C under loose contact conditions, respectively.

To compare the activity of the CeMn nano-catalyst with other work, the relevant dates reported in literature are listed in Table 4. Three-dimensionally ordered macroporous (3DOM) MnO₂ composites exhibit the best activity for soot combustion ascribed to their interpenetrating well-ordered macroporous structure and large surface area. The catalyst prepared in this work demonstrates higher activity than other catalyst with the similar component.

Table 4 The catalytic activities for soot combustion over CeMn-type catalysts in literature

Entry	Catalyst	Reactant gas	Contact	Tm or T50 or Tp (°C)	References
1	Ce0.7Mn0.3O2−δ	Air	Tight	392	45
2	Mn0.5Ce0.5Oδ (3DOM)	0.2% NO + 10% O2	Loose	358	29
3	MnO2	500 ppm NOx + 5% O2	Loose	430	46
4	MnOx–CeO2	10% O2	Loose	503	16
5	Mn–Ce oxide	Air flow	Loose	475	47
6	MnOx–CeO2	Air flow	Loose	542	48
7	CeMn-600	21% O2 + 79% N2	Loose	418	This work

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4. Conclusions

In summary, MnO₂ nanocomposite materials were successfully prepared via a facile hydrothermal method starting from KMnO₄. The hydrothermal process of KMnO₄ was accelerated by the CeO₂ and Co₃O₄, which could contribute to the hydrolyzation of KMnO₄. CeO₂ could lead to the shrink of the ceria lattice in CeMn. However, after calcination at 600 °C, there is an expansion in ceria lattice of CeMn-600. The existences of more Ce³⁺ bring about more oxygen vacancies that are beneficial to soot oxidation. The CeMn-600 exhibit superior catalytic performance toward soot oxidation compared with pure α-MnO₂. Therefore, the proposed hydrothermal route for the synthesis of MnO₂ nanocomposites could be a promising way to prepare catalysts for soot combustion.

Acknowledgements

The work presented above was supported by National Science Foundation of China (No. 21306082), R&D Project for Environmental Protection of Jiangsu of China (No. 2015002), the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (ZK201305) and the Research and Innovation Training Project for Graduate in General Universities of Jiangsu Province (No. 201310291004Z).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02045c

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